Rotary machines are widely used in industrial applications for converting electrical energy into rotational movement (in the case of a motor) or for producing electrical energy from rotational movement (in the case of a generator). To a large extent the requirements for the design of electric motors and generators are the same and so only the case of electric motors will be described in detail hereinafter.
Many applications require the provision of a variable-speed rotary machine. Although a large number of different types of electrical machine are known, the most widely used motor type for high power applications are AC synchronous and AC induction motors. Both AC synchronous and AC induction motors comprise a central rotor which is disposed about an axis of rotation within a stator assembly. A magnetic field rotating at synchronous speed is produced by passing current along electrical conductors arranged in a pattern around the stator. The magnetic field penetrates the air gap between the stator and the rotor and in turn causes the rotor to rotate. The field-generating electrical conductors are usually stator winding elements (variously called “windings”, “field windings”, or “coils”) and typically consist of many turns of electrical wire wrapped around a former or stator core. However, straight bar-shaped field-generating conductors may also be used in some situations, their ends being connected together across the stator ends to form the required electrical circuits.
To generate the high flux densities needed in many applications the windings are usually grouped together in a fixed configuration in a predetermined number of phases. Each winding in a given phase is driven with the same current waveform if the windings are connected in series, or with the same voltage waveform if they are connected in parallel.
The windings in a machine stator are normally shown diagrammatically in the shape of a diamond as illustrated in FIG. 7 of the accompanying drawings. Each winding comprises an input terminal 701 at the start of the conductor used to form the winding. The conductor is arranged in a loop having portions 702 to 703 to 704 to 705 to 706 to 707 and back to 702, and this loop is usually repeated a number of times to form a coil. Finally, an output terminal 708 is provided at the end of the coil. Each conductor may comprise a bundle of many individually insulated copper wires, the bundle being covered with an insulating sleeve.
Two side portions 703 and 706 of the winding lie parallel to the axis of the machine and are responsible for producing the rotating flux. The remaining portions 702, 704, 705, and 707 are used to interconnect the two sides 703 and 706 and do not produce useful flux. These portions are commonly referred to as the “coil overhangs” or “end windings”.
To group the individual windings together into phases, the stator includes conductive links that connect the overhanging ends of the windings after they have been produced. This connection process is costly and time consuming. The connections are normally provided at one end of the stator and the overhang connections can add significantly to the overall length of a complex high power machine.
A problem with prior art machines of this type arises if a short circuit occurs across the windings, or a wire inside a winding breaks, giving an open circuit. Short-circuiting can also occur due to faults external to the windings. When a short circuit fault arises the machine may need to be stopped to permit removal of the short circuit, since a closed loop around a winding will generate an undesirable braking torque that will prevent correct operation of the machine.
Similarly, a wire breaking inside a winding will affect all the associated windings and will require the machine to be stopped for repair.
For steady state operation, crude control of the machine is possible by connecting the windings together to form three phases and applying a respective phase of a three-phase electrical supply to each of the winding phases. The machine will rotate at a steady speed determined by the frequency of the waveform—usually 50 or 60 Hz—and the arrangements of the windings.
If a variable speed is needed the speed of rotation of the magnetic field must be varied by changing the waveforms applied to each of the phases. This is performed by a control circuit—often referred to as a converter—which receives a voltage supply as an input and produces as an output the waveforms needed for each phase of the machine. Many different converter circuits are known in the art and further details of operation of converters are not necessary for understanding of the present invention.
For a machine made with three phases, if a fault occurs in one phase the machine will be unable to operate either on a fixed or a variable frequency supply.
To avoid this problem, machines can be made with more than three phases, e.g. six or nine phases. For machines with increased numbers of phases, if a fault occurs in a phase of the machine, the converter can automatically isolate a phase and in many cases continue to run the machine at a reduced torque. However, this is a crude form of protection, as a fault in a single winding will require a complete machine phase to be isolated.
Alternatively, it has been proposed to incorporate one or more switches into the wiring that connects together the windings into phases. These switches—which can simply be switched between a conductive state and a non-conductive state—either connect or isolate windings from their respective phases. The switches are closed for normal operation, but when a fault is detected the switches are opened as required to isolate the faulty winding or windings. The switches do not change the shape of the current flowing in the windings—they can only switch the current on or off.
The invention aims to provide an electrical rotating machine in a more useful and convenient form by combining electric power switching and busbar technology to facilitate reduction of stator length, and to enable reconfiguration of stator windings and change of their flux generating characteristics while the machine is operating.